1. Field of the Invention
The present invention relates to novel ferroelectric liquid crystal devices.
2. Discussion of the Background
For display purposes, liquid crystal (LC) and liquid crystal polymer (LCP) films are typically sandwiched between two transparent, conducting electrodes (usually, indium tin oxide, ITO) set apart at some fixed distance by a spacer. A voltage is applied across the electrodes, inducing a field in the film. Fields in the film are calculated by dividing the voltage by the spacer distance and are typically on the order of 10.sup.5 V/cm for LCPs. Changing the voltage across the electrodes can cause LCs and LCPs to switch. To monitor the effects of changing the potential, a polarizer is set parallel to the film; switching can be observed as the change in light intensity transmitted through the polarizer-film set-up.
It was first shown by Meyer et al (R. B. Meyer et al, J. Phys. Lett., vol. 36, L69 (1975)) that the smectic-C phase of chiral molecules (SmC*) is a ferroelectric liquid crystal (FLC). This was followed by the demonstration of fast electro-optic switching (in the range of .mu.s) and optical bistability in surface-stabilized ferroelectric liquid crystal (SSFLC) displays (N. A. Clark et al, Appl. Phys. Lett., vol. 36(11), 899 (1980)). In the material is enclosed between two conducting glass substrates whose surfaces are "pre-treated" to cause the molecules to lie in the plane of the substrate. Thus, the molecules are held in one of their two electrically addressable, optically different states through their interaction with the surfaces of the cell. This memory effect in combination with relatively fast switching has made FLCs attractive for electro-optic applications.
Other electro-optic effects in FLCs using birefringence are the transmit scatter mode (TSM) (K. Yoshino et al, Jap. J. Appl. Phys., vol. 2, L385 (1984)), deformed helix ferroelectric (DHF) (J. Funfschilling et al, J. Appl. Phys., vol. 66(8), 3877 (1989); and L. Beresnev et al, Liq. Cryst., vol. 5, 1171 (1989)), short pitch ferroelectric (SPF) (J. Funfschilling et al, SID 90 Digest, 106 (1990)) and soft-mode effect (S. Garoff et al, Phys. Rev. Let., vol. 38, 848 (1977)). While these effects are observed in sandwich cells with a planar alignment of the molecules, there have also been recent reports on different sample confinements like free-standing SmC* films (G. Hauck et al, Ferroelectrics, vol. 122, 253 (1991)) and homeotropically aligned sandwich cells (M. Ozaki et al, Japan. J. Appl. Phys., vol. 30(9B), 2366 (1991)); in the latter case, a change in transmission could be observed with oblique incident light.
As noted above, ferroelectric LCP display devices generally use the SSFLC geometry, in which the material is enclosed between two conducting glass substrates whose surfaces are "pretreated" to cause the molecules or mesogenic groups to lie in the plane of the substrate. Although the switching times of such SSFLC devices are on the order of .mu.s, devices with even faster switching times are desired. Further, such SSFLC devices are required to have at least two substrates and an intervening layer of LCP (having a thickness of about 2-4 .mu.m). Thus, such SSFLC devices typically have a thickness on the order of about 4 .mu.m, and thinner devices are desired.
One constraint on the thickness of SSFLC devices arises from the surface irregularities present on the top and bottom plates. Thus, the thickness of the LC layer between the two plates must be at least 2-4 .mu.m to prevent a short circuit between the top and bottom surfaces.
Thus, there remains a need for thin ferroelectric LCP devices which exhibit fast ferroelectric switching times. In particular, there remains a need for devices in which short circuiting between the top and bottom plates sandwiched about a LC film is not a problem.
In addition, pyroelectric materials are useful for use in infrared (IR) sensors or imaging devices. Currently, the limitation on IR imaging capability (a combination of sensitivity and resolution) is the thickness of available pyroelectric materials. In particular, there is a need for materials which exhibit pyroelectric properties even at thicknesses below 10 .mu.m. However, currently available materials lose their pyroelectric properties at thicknesses below 10 to 15 .mu.m.
Thus, there remains a need for materials which exhibit pyroelectric properties at thicknesses lower than 10 to 15 .mu.m. In particular, there remains a need for materials which exhibit pyroelectric properties at such low thicknesses and are suitable for use in IR sensor and/or imaging devices.
Langmuir-Blodgett (LB) films of LCP and LC oligomers have been reported (see: Z. Ali-Adid et al, Langmuir, vol. 7, 363 (1991) and Penner et al, Macromolecules, vol. 24, 1041 (1991)). However, to date, there are no reports on the ability of a LB film of a LCP to undergo electric field-induced ferroelectric switching.
In addition, pyroelectric materials based on ferroelectric polymers have been reported (Ding-Quan et al, IEEE Transactions on Electrical Insulation, vol. 23, p. 503 (1988)), and there are also reports of the pyroelectric properties of LB films (see: Jones et al, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 35, p. 737 (1988) and Roberts et al, Thin solid Films, vol. 180, p. 211 (1989); However, to date there are no reports of any LB films of a LCP exhibiting pyroelectric properties.